1. Field of the Invention
The present invention relates generally to the field of guidewires used to diagnose and treat maladies in humans and more specifically to pressure-sensing guidewires used in intravascular procedures.
2. Description of the Related Art
It is often desirable to determine the severity of a stenosis or occlusion in the coronary arteries by measuring the pressure distally and proximally of the stenosis or occlusion. Devices today that are used for this purpose include catheter like members with some type of pressure-sensing device incorporated therein. Such devices are often referred to as a pressure-sensing guidewire since they can provide the dual function of guidewire and a pressure measuring device. Many of these devices today are constructed by incorporating a piezo-resistive pressure sensing device towards the distal end of a hollow guidewire body. Three electrical wires are then run the length of the hollow guidewire in order to connect the piezo-resistive pressure sensing device to the proper measurement instrumentation.
One problem associated with the currently available pressure-sensing guidewires is the cost to manufacturer such a device. These devices can be up to 10 times more expensive to manufacturer than a standard guidewire and up to 20 times more expensive than a standard catheter with invasive blood pressure sensor. The main reason for the high cost is the piezo-resistive device itself and the labor required to run electrical wires through the length of guidewire and terminate them on the proximal end of the wire. Thus adoption of the current pressure-sensing guidewires by medical professionals is inhibited due to the large cost difference between these devices and standard guidewires.
A second problem associated with the currently available pressure-sensing guidewires is that the accuracy of the piezo-resistive pressure sensor inside the guidewire is not as accurate as a standard invasive blood pressure sensor. Due to limited space inside the guidewire, the piezo-resistive pressure senor must use a half bridge design in order to minimize the number of electrical wires that must be run the length of the guidewire. As a result, the currently available pressure sensing guidewires have reduced zero drift stability and increased susceptibility to thermal variations over standard full bridge invasive blood pressure sensors.
A third problem associated with the currently available pressure-sensing guidewires is reliability of such devices. These devices have electrical wires running the length of the guidewire that terminate to the piezo-resistive sensor on the distal end and to an electrical connector on the proximal end. These electrical connections must be sealed from the surrounding fluid in the body and any tiny breach will cause errors in the pressure measurement. As a result, there is high rate of failure with such a device because of the number of electrical interconnects and their proximity to fluids.
A guidewire with a single fluid filled lumen from the distal end to the proximal end, with a pressure transducer attached at the proximal end would solve the three problems mentioned above because it does not require electrical wires to run the length of the guidewire 10 and a full bridge, low cost pressure transducer can be used at the proximal end of the wire. However measuring pressure through a fluid filled tube creates distortions in the pressure waveform that creates measurement error because the pressure exerted on one end of the tube is no longer directly proportional to the pressure measured on the other end of the tube. In order to accurately measure pressure through a fluid filled tube, these distortions created by the fluid filled tube must be corrected for.
As shown in FIG. 1, when pressure is applied to one end 1 of a fluid filled tube 2 having a pressure transducer 3 on the other end 4 of the tube 2 the relationship between pressure p(t) and an electrical output E of the pressure transducer 3 is governed by the following differential equation:
            E      ¨        +                  c                  ρ          ⁢                                          ⁢          LA                    ⁢              E        .              +                  k                  ρ          ⁢                                          ⁢          LA                    ⁢      E        =            K              ρ        ⁢                                  ⁢        L              ⁢          p      ⁡              (        t        )            where ρ is the density of the fluid 5 in the tube 2, L is the length of the tube 2, A is the cross-sectional area of the inner diameter of the tube 2, K is a constant related the pressure transducer 3, k is a constant related to elastic forces inside the tube 2 and c is a constant related to the fluid column velocity. An alternative form of the equation above is:
            E      ¨        +          2      ⁢              ζω        n            ⁢              E        .              +                  ω        n        2            ⁢      E        =            K              ρ        ⁢                                  ⁢        L              ⁢          p      ⁡              (        t        )            where ωn is the natural frequency of the lumen filled tube 2 and pressure transducer 3, and ζ is the damping ratio of the lumen filled tube 2 and pressure transducer 3.
Accordingly, there is a need for a device and system that can function both as a pressure-sensing device and a guidewire but measures pressure by using a fluid filled lumen inside the guidewire and corrects for the errors that are created when pressure is measured though this fluid filled lumen or tube.